In recent years, electronic equipments have generally been required to offer lighter weight and more compact size. In accommodation of this, although the printed wiring boards used inside these equipments predominantly consist of 4-10 layers, in order to accommodate high-density mounting, efforts are being made to increase pattern fineness, and further to reduce mounting height and employ a built-up constitution.
Printed wiring boards are required to have low dielectric dissipation factor as well as be stable over a wide temperature range and wide frequency band in order to inhibit transmission loss when used in high-frequency circuits and ensure stable operation of the circuit in their temperature range and frequency bands. In addition, since materials having large dimensional expansion and contraction rates are frequently used for the surface built-up layer in built-up boards enabling low mounting height and high density mounting, they are susceptible to the occurrence of considerable warping due to contraction during curing and cooling of the surface built-up layer. Consequently, the printed wiring boards and multi-layer wiring boards on both sides of the inside core layer are required to be thin and have high rigidity. In addition, accompanying the use of a lead-free material for a solder, the melting point of the solder is expected to be higher, thereby resulting in the need for higher moisture absorption-heat resistance and reliability.
Thus, the materials for printed wiring boards are required to adequately possess all of the characteristics of low dielectric dissipation factor, high modulus of elasticity, high heat resistance, low moisture absorption and high glass transition point (Tg).
In addition, these materials are also required to be flame retardant in consideration of safety. Consequently, attempts have been made to ensure flame retardancy by combining the use of halogen-based flame retardants, antimony compounds or phosphorous-based flame retardants. In recent years, however, there has been a growing trend towards controls on substances used in consideration of environmental pollution and toxicity, and the toxicity and carcinogenicity of organic halogen substances such as dioxin in particular have become a problem. Consequently, there is a strong need to reduce or eliminate halogen-containing substances, and according to JPCA standards, halogen-free materials are defined as having individual contents of halogen elements being 0.09% by weight or less. In order to satisfy this standard value, it is necessary to suppress the content of each halogen element in the resin used to 0.25% by weight or less even in the case in which the halogen content in the resin is at the highest allowed level for the lowest amount of the resin used and the minimum resin weight being about 38% of the total weight of the prepreg.
In order to satisfy the above requirements, materials in which an epoxy resin, a polyimide resin or an isocyanate resin is improved or modified have been researched. Among these, resins having a dihydrobenzoxazine ring have superior characteristics such as low dielectric dissipation factor, high modulus of elasticity, high heat resistance, low moisture absorption, high Tg and high flame retardancy, so that they have been attempted to use in a substrate for wiring boards and so forth. However, since resins having a benzoxazine ring have a rigid skeleton, they do not exhibit toughness. Consequently, they are susceptible to the occurrence of inner layer separation during blank die processing, and have poor discharge of cuttings during small diameter drilling, resulting in the problem of susceptibility to occurrence of breakage of the drill bit. In order to solve such problems, it is necessary to blend epoxy resin into the resin composition at 40 parts by weight or more based on 100 parts by weight of the organic solid components as disclosed in Japanese Patent Laid-Open Publication No. 11-158352. However, when such epoxy resin is formulated in relatively large amounts, problems occur including loss of the inherent characteristics of low dielectric dissipation factor, high modulus of elasticity, high heat resistance, low moisture absorption, high Tg and superior processability, while also preventing the attainment of V-0 flame retardancy as specified in standard UL94 pertaining to flame retardancy in thin materials.
Consequently, a printed wiring board material is required that has low dielectric dissipation factor, high modulus of elasticity, high heat resistance, low moisture absorption, high Tg as well as a halogen and antimony compound content in a thermosetting resin composition of 0.25% by weight or less, and flame retardancy of V-0 in standard UL94.